1. Field of the Invention
The present invention relates, in general, to a solid electrolytic capacitor and, in particular, to an improved solid electrolytic capacitor including a capacitor element with a capacitance that is increased by enlarging the space occupied by the capacitor element within a limited space of the epoxy case, and an anode lead frame which can minimize the heat transfer to the capacitor element during an assembly of a capacitor element and a lead frame, simplifying the production of the solid electrolytic capacitor.
2. Description of the Prior Art
A solid electrolytic capacitor is an electronic device for storing an electric charge, preventing the passage of a direct current, and passing an alternating current. Among various solid electrolytic capacitors, a tantalum capacitor is most widely applied to general industrial machinery, and to an application circuit used in a low rated voltage range. In particular, the tantalum capacitor is used to reduce a noise of a circuit or a portable communication apparatus in which a frequency characteristic is important.
The tantalum capacitor 100, as shown in FIGS. 1 to 4, comprises a capacitor element 110 consisting of dielectric powder which determines the capacitance and characteristic of a capacitor, an anode lead frame 130 and a cathode lead frame 140 connected to the capacitor element 110 so as to easily mount the capacitor on a printed circuit board (hereinafter referred to simply as a xe2x80x98PCBxe2x80x99), and an epoxy case 150 for protecting the capacitor element 110.
A process of manufacturing the tantalum capacitor 100, comprises the steps of pressing tantalum powder into rectangular parallelepiped-shaped pellet, sintering and degassing the rectangular parallelepiped-shaped pellet, anodizing the pellet to form tantalum oxide (Ta2O5) layer on the exposed tantalum surfaces, infiltrating a manganese nitrate solution into the pellet, and thermally decomposing the infiltrated pellet to form a manganese dioxide layer, that is, a solid electrolyte on a surface of the resulting pellet.
A process of connecting the anode lead frame 130 and the cathode lead frame 140 to the capacitor element 110 thus manufactured comprises the steps of welding a rod-shaped anode wire 120 protruding by a predetermined length from a lateral side of the capacitor element 110 to a plate-shaped anode lead frame 130 by an electrical spot welding process to form an anode terminal, and soldering the cathode lead frame 140 to an external surface of the capacitor element 110 using a conductive adhesive such as carbon or silver powder coated on the external surface of the capacitor element 110 to form a cathode terminal (See Japanese Laid-Open Patent Publication No. 5-335189 of Honda Hisafumi et al.). Thereafter, the capacitor element 110, electrically connected to the anode lead frame 130 and cathode lead frame 140 is molded with epoxy powder in an encapsulating step so as to form an epoxy case 150 for protecting the capacitor element 110, and subjected to a marking step which ends the manufacturing process of the capacitor 100.
However, the conventional process of welding the anode wire 120 to the anode lead frame 130 while they are in contact with an upper and a lower electrodes 161 and 162, indispensably comprises a bending step of forming a flat pressed surface 122 on an external side of the anode wire 120 before welding of the anode wire to the anode lead frame in order to prevent shaking occurring in welding, and to increase a contact efficiency between them. The conventional process is thus disadvantageous in that an external mechanical impact readily occurring in the bending step is transferred through the anode wire 120 to the capacitor element 110 destroying the dielectric layer. As a result, the electrical property of the capacitor, for example, an LC value is degraded. In addition, the production cost of the capacitor is increased owing to the bending step.
In addition, in case that the anode wire 120 is welded to the anode lead frame 130 with a metal such as lead or tin, said metal can be melted due to the high temperature generated when mounting the capacitor. Thus, a broken electrical connection can occur.
Furthermore, a conventional process of soldering an external lower side of the capacitor element 110 to an upper side of the cathode lead frame 140 with a conductive adhesive is disadvantageous in that the space occupied by the capacitor element 110 within a limited space of an epoxy case 150 is relatively small, and a volume of the capacitor element 110 is small, thereby limiting capacitance of the capacitor 100 and the increasing impedance.
Meanwhile, the anode wire 120 of the capacitor element 110 may be welded to the anode lead frame 130 by a laser welding process instead of the electrical spot welding process as disclosed in Japanese Laid-Open Patent Publication No. 8-195330 by Mitsui Koichi et al. More specifically, a V-shaped notch part 132 is formed on the anode lead frame 130, the anode wire 120 of the capacitor element 110 is mounted on the notch part 132, and portions of the frame 130 located at both sides of the anode wire 120 are melted by a laser beam to weld the anode wire 120 to the anode lead frame 130, as shown in FIGS. 5a to 5c. 
However, when the anode wire 120 is welded to the anode lead frame 130 by the laser beam, the welding process is very complicated because the laser beam is simultaneously irradiating two portions of the frame 130 located on both sides of the anode wire 120.
In addition, because the portions irradiated by the laser beam are restricted to a cut section of the notch part 132 corresponding to the thickness of the anode lead frame 130, an area for welding the anode wire 120 to the anode lead frame 130 is small, and so the laser output of the laser welding machine must be increased in order to increase welding efficiency. At this time, a spark occurring during the laser welding may reach the capacitor element 110, damaging the capacitor element 110.
Moreover, the external surface of the anode lead frame 130 has a high absorbability of the laser beam because the external surface usually has a dark gray color, and so the welding characteristic thereof is excellent, but the notch part formed in a shape of xe2x80x98Vxe2x80x99, having a color of an inner metal of the anode lead frame 130 has a poor absorbability of the laser beam and a high reflectivity against the laser beam, and so the welding characteristic thereof becomes poor. Accordingly, the laser output of the laser welding machine must be increased in order to improve the welding efficiency, and thus consumption of electricity is increased, and heat impact and sparks transferred to the capacitor element 110 are increased, thereby increasing the damage to the element.
Therefore, it is an object of the present invention to avoid the above disadvantages, and to provide a solid electrolytic capacitor, which can avoid a bending process, minimize the heat transfer to its capacitor element in order to obtain a stable electrical characteristic, increase its operational reliability, and reduce its production cost owing to a simplified production process of the solid electrolytic capacitor.
It is another object of the present invention to provide a solid electrolytic capacitor, which can sufficiently enlarge the capacitance of its capacitor element by increasing the space occupied by the capacitor element within a limited space of an epoxy case.
It is still another object of the present invention to provide a solid electrolytic capacitor, which can improve welding efficiency between a lead frame and an anode wire by preventing a shaking of the anode wire.
Based on the present invention, the above objects can be accomplished by a provision of a solid electrolytic capacitor, comprising a capacitor element; an anode wire extending from a first side of the capacitor element by a predetermined length; an anode lead frame having a groove at a first end thereof for mounting an end portion of the anode wire thereon and a second end thereof for mounting on a PCB; a cathode lead frame having a first end attached to an external surface of the capacitor element and a second end for mounting on the PCB; and a mold case, preferably an epoxy case, covering the capacitor element, the anode lead frame, and the cathode lead frame. In the capacitor, the anode wire is welded to the anode lead frame by melting a portion of the anode lead frame in contact with the end portion of the anode wire positioned on the groove, using a heat source.